Control means for rectifier tubes



NOV. 19, 1935. D, v EDWARDS 2,021,482

CONTROL MEANS FOR RECTIFIER TUBES Filed Dec. 10, .1933

08 I KZVENTORI V M MvM ATTORNEYS.

Patented Nov. 19, 1935 UNITED STATES PATENT OFFICE CONTROL MEANS FORRECTIFIER TUBES Donald V. Edwards, Montclair, N. J assignor toElectrons, Inc. of Delaware, a corporation of Delaware ApplicationDecember 10, 1932, Serial No. 646,625 11 Claims. (01. 175-363) Thisinvention relates to the control of gaseous wise, a voltage in M suchthat the grid of tube 6 discharge rectifier tubes having control grids,will be negative when its plate is positive. This i. e., tubes-whereinthe starting of an arc discharge w p v t t a fr m sta t n b th tub s iscontrolled by a grid element and extinguished and no current will flowthrough either tube. If

by reducing the plate potential to zero. the tubes are of the typerequiring positive bias 5 The object of the invention is to providemeans to start, the operation will be the same except for controllingthe tube, which means shall rethat no current will flow until contact IIis moved quire minimum energy without sacrifice of deo point 9 TheVOltage generated W then m pendability. the grids more positive than thetrigger point A further object is to provide an improved grid on therespective half cycles when the plates are 10 transformer for suchtubes, positive and current will flow through the load.

The invention will be described with reference A characteristic of thegrid controlled gaseous to the drawing in which discharge tube is thatwhile the discharge is pres- Fig. 1 shows schematically a typicalcircuit arent the gird if near cathode potential will draw rangementemploying the invention, and energy from the plate circuit and feed itback 15 Fig. 2 is a diagram showing the relation of variinto he gridcircui O din ily he r y o ous voltages in the circuit shown in Fig. 1.fed back builds up magnetic flux in the core of the Referring to Fig. 1,an alternating current transformer. When the discharge in the tubesource I energizes transformer 2, the secondary stops at the end of theactive cycle, this flux repcoil 3 of which is connected to the plates 4,4 of resenting stored energy tends to generate a volt- 20 the gridcontrol rectifiers 5, 6 in a full-wave cirage in the transformerprimarycausing current cuit which includes a load shown schematically at toflow back into the control source; .and in the I, and connected betweenthe midpoint of the secondary, tending to influence the gridpotentransformer secondary and the midpoint of the tial. This is shownin Fig. 2 where curve It is secondary 8 which feeds the cathode circuit9. the potential from the grid to the center tap in 25 At I0 is a sourceof control energy having internal a well designed circuit employing atransformer resistance which, for the purpose of describing of lowenergy storing capacity. If the circuit this invention, may beconsidered as a resistor employs the usual design of transformer, theconnected across the source of potential I. The curve will not followthe solid portion of curve 3 invention applies equally well if insteadof resistit which lies to the right of the negative peak but ance In thesource of control energy is the variinstead will follow some curvesimilar to the dotable current in the plate circuit of a vacuum tube tedportion shown at n. Curve d is the plate or any of the commonly usedbridge circuits, since, voltage of tube 5, and m the grid potential whenin both cases, the control circuit may be considno feed-back is present,as when the grid just ered as having internal resistance equivalent toprevents starting. It can be seen that once the 35 resistance b, c. Thecontrolling potential across tube passes current, control on thesucceeding leads I I and [2, at least the former of which prefcycleswill be affected. On the second half cycle erably is variable, operatesthe primaries of transshown, the grid is more negative due to storedformers l3 and M. The secondary of transenergy by the amount p. Thiswill not affect former I3 is connected between the center tap of tube 5because its plate is negative but an approx- 40 the filament winding andgrid of tube 5 with a imately equal voltage in the opposite directionseries resistance l5 which may be an external rewill be induced into thegrid of 6 making it posisistance, or, preferably, the internalresistance of tive and allowing it to start-if the amount p is thesecondary winding of transformer l3. sufficiently large, for instance,if the curve fol- Transformer I4 is connected to tube 5 in a similarlowed the dotted portion n. In a half-wave cir- 45 manner throughresistance It. If the sliding cuit containing only tube 5, control onthe next contact H is .at b, no voltage is generated and the positivecycle would be influenced to the extent grids assume the potential ofthe center tap of shown at s, tending to prevent discharge but inthefilament, allowing both tubes to start each effective unless a curvesimilar to u is followed. cycle and during the discharge pass currentThus when the usual audio-frequency trans- 50 through load 'I. Ifcontact H is moved slightly formers are used for transformers l3 and I4sevtoward 0, an A. C. voltage is impressed on the eral hundred times asmuch energy input will be primaries of transformers l3 and I4,generating required to control the tubes than would be neea voltage inl3 such that the grid of tube 5 will essary if direct current whereapplied directly to be negative when the plate is positive, and, likethegrids. Moreover, there is a strong tendency 55 for the tube that shouldbe out to pass current Clllllllg an occasional cycle, or to make thecut-off point indefinite, there being guite a large range where thetube, instead of going out sharply, will flicker periodically.

I have found that these difiiculties can be overcome by the use ofspecial grid transformers making it possible to get sharp control withmuch less power input. To do this the transformer is designed to storeas little energy as possible. This insures that the available controlenergy is used mainly for supplying transformer losses and generatingvoltage on the grid of the tube rather than in overcoming fed-backenergy. Specifically, I make the transformer according to the followingmethod:

The secondary l7, I8 is wound to give the desired voltage on the grid;present-day tubes require about 4 volts to allow for variation betweentubes plus variation of tubes with life. The smallest possible wire isused, preferably small enough to give an internal resistance equal tothe highest grid resistance compatible with good tube operation. If, forinstance, the grid current limit of control tubes available is 10microamperes or under, a resistance of 10,000 ohms will be satisfactory,since the variation between tubes due to varying grid currents will notamount to more than volt.

It is necessary to know from the tube data approximately how muchcurrent will return through the grid circuit after the discharge hasstarted with this value of resistance. A typical value for present-daytubes is 300 micro-amperes peak. The iron circuit of the transformer isdesigned so that the energy stored in it as magnetic flux by this amountof current flowing in the secondary can be substantially dissipated bythe primary windings in less time than the interval during which bothtubes are out at the end of each active cycle. The mathematicalintegration involved in determining this time is too complicated tooffer much practical help. The simplest solution at present seems to beto take small increments of time and calculate point by point the gridvoltage during and after an operating cycle. A sufficiently accuratesolution may be found in the following manner: From the tubecharacteristic it is fairly simple to calculate the current flowing inthe transformer at the end of an active cycle. After the dischargeceases a current will flow for an instant giving the same number ofampere turns on the core of the transformer. Then over a small incrementof time, say 5, the voltage necessary to make this primary current flowin the transformer primary circuit is determined from the circuitconstants and in ternal resistance of the control source. The decreasein flux in the core necessary to generate this voltage is easilycalculated and thus the flux at the end of the small increment of timedetermined. From the magnetization curve of the transformer, the currentflow corresponding to this flux is determined. Using this value as theinitial current for another small increment of time the same method isfollowed for it and so on until a complete cycle is obtained. Thenplotting the potential of the grid after an active cycle will show theeffect of the stored energy on the control for the next active cycle. Ifsuch a curve is made for a number of values of internal resistance ofthe control source, it is found that de creasing resistance prolongs thetime necessary for dissipating energy.

The energy stored in the core will be reduced if an iron which has asharp magnetic saturation point is used and is operated to give thedesigned secondary voltage at just under saturation. It also requiresthat the finest possible wire be used for the secondary, and the windingfactors of both primary and secondary be made as high as possible toreduce the weight of iron used. This will result in a transformer of lowefficiency and high magnetizing current. However, in spite of this theenergy required to control the tubes is greatly reduced and thesensitivity increased because the control source does not have toovercome the energy fed back from the control tubes during the operatingpart of the cycle.

Less power is required if two transformers are used with two separatecores; one for each tube rather than both secondaries on one corealthough the total losses are higher.

Most applications of control tubes are for 60- cycle circuits, in whichcase it is possible to use transformers having high core losses. Thiswill result in the transformer having a low cut-off frequency but thestability of the circuit will be greatly improved because transientspicked by leads II and I2 will not build up voltage across thetransformer and allow false operation of the tube. It is then onlynecessary to shield the leads from the transformer to the grid and alsothe grids themselves by any suitable means, as for example the metalliccasing Hi, to prevent picking up stray fields. Some of the grid currentbefore discharge of the grid control rectifier tubes is highly irregularin wave form. If the core losses are relatively high, the impedance ofthe transformer to these waves is reduced and tube stability increased.

In view of these considerations it would seem that the real limit to thevoltage step-up ratio of transformers for controlling grid rectifiers isfixed by the current available to supply core loss and magnetizingcurrent. This sets a premium on building the transformer just as smallas possible, which is in direct contradiction to present conceptions ofgood transformer design. Transformers with as high as 1/200 voltagestep-up ratio have been operated satisfactorily, and there seems to beno reason at present why this value cannot be increased to 1/1000 ormore.

In some cases the magnetizing current of the transformers l3 and I4 willshift the phase of their secondary voltages. This can be compensatedfor, if necessary, by shifting the phase of the input energy to thecontrol source It! by any of the well-known means. In other cases,advantage can be taken of this phase shift to allow triggering the tubesbefore the crest of the plate voltage.

The invention has been described with respect to the all-on, all-offoperation of a full wave circuit. The principles apply to half wavecircuits in the same manner except that the time available for thecollapse of flux in the core may be greater. The principles also applywhere a phase shift control circuit is used to obtain graduated controlrather than the all-on, all-off circuits shown.

What I claim is:--

1. The combination with a grid controlled gaseous rectifier, of a gridtransformer supplying control voltage to the grid of the rectifier, andmeans for dissipating the voltage induced in said transformer by thecessation of the gaseous discharge.

2. The combination with a grid controlled gaseous rectifier of meansincluding a reactive element for applying an alternating control voltageto the grid, and means for reducing the current induced in said controlmeans by the action of the gaseous discharge.

3. The combination with a grid controlled gaseous rectifier of a gridtransformer supplying alternating control voltage to the grid of therectifier, and means for reducing the storage of energy in'thetransformer due to the cessation of the discharge.

4. The combination with a grid controlled gaseous rectifier of atransformer supplying alterhating control voltage to the grid of therectifier, the core of said transformer being adapted to be nearlysaturated in providing the grid current at the control voltage normallyrequired to start the gaseous discharge. 7

5. A grid transformer for use with grid controlled rectifier tubeswherein the magnetic core is so small that the energy stored from anoperating cycle will be substantially dissipated before the start of thenext operating cycle.

6. A transformer for use with grid controlled gaseous rectifier tubeshaving a secondary wound with wire of approximately the size to carrythe designed grid current before starting, and having the core designedto be worked at a flux density slightly under the saturation point inproviding the normal grid control potential.

'7. The method of operating a grid controlled gaseous rectifier with analternating current source of control voltage, said rectifier havingreactance in its control grid circuit, which consists in making the saidreactance a minimum consistent with its providing the necessary gridvoltage to start the gaseous discharge, and dissipating before thesucceeding control period a substantial part of the grid voltage inducedby the cessation of the discharge.

gaseous rectifier of a transformer having its primary connected to asource of control energy for supplying alternating voltage to the gridof the rectifier and having its secondary connected in the grid circuitof the rectifier, said grid cir- 5 cuit being conductively insulatedfrom said source, and the core and secondary of said transformer beingadapted to give a high ratio of resistance to reactance.

9. The method of operating a grid controlled 10 gaseous rectifier withan alternating current source of control voltage, said rectifier havinginductive impedance in its control grid circuit, which consists inmaking the reactive component of said impedance a minimum and itsresistance 15 component a maximum consistent with its providing thenecessary grid voltage to start the gaseous discharge, and dissipatingbefore the succeeding control period a substantial part of the gridvoltage induced by the cessation of the dis- 20 charge.

10. A grid transformer in combination with grid controlled rectifiertubes wherein the magnetic core of said transformer is so small that theenergy stored from an operating cycle will be sub- 5 stantiallydissipated before the start of the next operating cycle, the secondarycircuit of said transformer having a high ratio of resistance toreactance.

11. A transformer of small core dimensions for grid controlled rectifiertubes, together with resistance in series with the grid of a valuerelative to the core of said transformer to permit developing thedesired starting control voltage on the grid but limiting the gridcurrent drawn from the operating discharge to prevent storage of moreenergy in the said core than can be substantially dissipated betweenoperating cycles.

DONALD V. EDWARDS.

